This invention relates to thermostatically controlled circuits for energizing electrical resistance heaters and, more particularly, to thermostatically controlled circuits for energizing electrical resistance heaters in an apparatus for melting and dispensing thermoplastic material. Specifically, the invention is directed to a fail-safe thermostatically controlled circuit for energizing electrical resistance heaters in high-throughput thermoplastic material melting and dispensing apparatus.
Thermoplastic material is commonly used today as an adhesive. Consider, for example, the use of thermoplastic material as an adhesive for constructing packages. An article is initially placed in a partially erected carton. Next, molten thermoplastic material is applied to the carton flaps which are then folded for adhesively joining the flaps together so that the article is sealed in the carton once the molten thermoplastic material solidifies. Hot melt adhesives are also used during the assembly of many products, such as automobiles, electronic components, electrical equipment, appliances, furniture, aircraft subassemblies, and products wherein a metal-to-metal bond is needed.
Thermoplastic material is usually supplied in the form of solid chunks which must be melted to the molten state before the thermoplastic material is used. Various types of apparatus are known for melting solid thermoplastic material to the molten state and for dispensing the molten thermoplastic material to an applicator or dispenser. One such apparatus suitable for melting and dispensing thermoplastic material is shown in U.s. Pat. No. 3,981,416 which is owned by the assignee of the present invention, and the disclosure of that patent is incorporated as if fully set forth herein.
The thermoplastic material melting and dispensing apparatus shown in U.S. Pat. No. 3,981,416 includes a hopper into which solid thermoplastic material is deposited. The hopper feeds the solid thermoplastic material to a grid melter. Electrical resistance heaters heat the grid melter for melting the thermoplastic material. The molten thermoplastic material flows through the grid melter to a reservoir. Electrical resistance heaters heat the reservoir for maintaining the thermoplastic material in the molten state. The molten thermoplastic material is pumped from the reservoir to an applicator or dispenser.
It is desirable to rapidly melt the solid thermoplastic material and to maintain only a small reservoir of molten thermoplastic material in order to reduce the length of time during which molten thermoplastic material is exposed to oxygen and heat so that degradation of the thermoplastic material is minimized. In order to rapidly bring the grid melter up to a temperature for melting the solid thermoplastic material and to rapidly bring the reservoir up to a temperature for maintaining the thermoplastic material in a molten state, the maximum temperature of the electrical resistance heaters which pass through the grid melter and which are disposed in the walls of the reservoir are capable of heating the grid melter and the reservoir to a temperature which generally exceeds the flash point of the thermoplastic material. The maximum temperature of the grid melter and the reservoir, for example, may be on the order of 800.degree. F., whereas the flash point of the thermoplastic material may be 650.degree. F. Consequently, thermostatically controlled circuits are used for energizing the electrical resistance heaters which pass through the grid melter and which are disposed in the walls of the reservoir in order to rapidly bring the grid melter up to the temperature for melting the solid thermoplastic material and to rapidly bring the reservoir up to the temperature for maintaining the thermoplastic material in a molten state but to prevent the temperature of the grid melter or the reservoir from reaching the flash point of the thermoplastic material.
It is generally desirable to operate the grid melter and the reservoir at different temperatures. In the case of many thermoplastic materials, solid thermoplastic material melts at one temperature, for example, 425.degree. F., and freezes at a lower temperature, such as 375.degree. F. In the case of such thermoplastic materials, it is preferable to operate the grid melter at a temperature slightly above the melting temperature and the temperature of the reservoir slightly above the freezing temperature. As a result, degradation of molten thermoplastic material in the reservoir is minimized. Therefore, separate thermostatically controlled circuits are provided for energizing the electrical resistance heaters which pass through the grid melter and the electrical resistance heaters which are disposed in the walls of the reservoir.
The thermostatically controlled circuits generally include thermostats of either the bimetal or the bulb type. One thermostat is located near the grid melter and another thermostat is located near the reservoir for sensing the temperature of the molten thermoplastic material. Each thermostat in turn energizes a contactor for connecting a power source to the electrical resistance heaters when the temperature of the molten thermoplastic material is below a preselected thermostat temperature setting.
In the case where bimetal thermostats are used, there is a possibility that the thermostats can fail, for example, the contacts of either bimetal thermostat can weld closed. The use of bulb thermostats is preferred since such thermostats per se are inherently fail-safe, that is, the gas within either bulb thermostat vents off if the bulb ruptures. Nevertheless, the bulbs generally actuate microswitches which in turn energize the contactors for connecting the power source to the electrical resistance heaters. There is a possibility that the microswitches can fail, for example, the contacts of a microswitch can weld closed. Furthermore, in either the case where bimetal thermostats are used or the case where bulb thermostats are used, there is a possibility that the contactors can fail, for example, the contacts of a contactor can weld closed or the spring which is used for opening the contacts when the contactor is de-energized can break. If a failure occurs, the electrical resistance heaters continue to be energized by the power source after the preselected thermostat temperature setting is reached. As a result, the temperature of the grid melter or the reservoir heated by the electrical resistance heaters rises above the preselected thermostat temperature setting toward the flash point of the thermoplastic material, thereby creating a risk of fire or explosion.
In view of the recognized failure problems, various fail-safe circuits have been proposed for avoiding the risk of fire or explosion if the thermostatically controlled circuit fails. However, fail-safe circuits heretofore used have the unfortunate limitation that they cannot be used in conjunction with thermostatically controlled circuits for the grid melter and the reservoir of high-throughput apparatus for melting and dispensing thermoplastic material which are energized at a high voltage and current.
Generally, the fail-safe circuits heretofore used in thermoplastic material melting and dispensing apparatus can be classified as being either manually resettable or disposable. Known manually resettable fail-safe circuits have a form similar to thermal overload circuits generally used in small electrical motors as shown in U.S. Pat. No. 2,223,729, for example. Disposable fail-safe circuits, on the other hand, include fuses having contacts joined by eutectic solder. A common characteristic of these fail-safe circuits is that current flows from the power source through the manually resettable or disposable element to the electrical resistance heaters. However, the manually resettable and disposable elements are capable of operating for prolonged periods of time without failure only at a nominal voltage and current, such as 120 volts and 10 amps. Consequently, fail-safe circuits heretofore used have been restricted to apparatus for melting and dispensing thermoplastic material having electrical resistance heaters energized by a 120 volt, 10 amp or other nominal power source. High throughput is not achievable under such conditions, that is, high throughput requires larger electrical resistance heaters energized by a high voltage and current, such as 240 volts and 15 amps. Heretofore, a fail-safe thermostatically controlled circuit for use in high-throughput thermoplastic material melting and dispensing apparatus has not been available so far as is known.